1. Field of the Invention
This invention relates to the extrusion of plastics, and more specifically to a method and apparatus for extruding bulky articles of thermoplastic while avoiding defects due to shrinkage.
2. Description of the Prior Art
As plastic products have become more widespread throughout society, extrusion methods have become an important technique for their industrial production. Ordinarily, plastics are extruded by passing heated, formable plastic articles through a die, and then allowing the extrudate to cool in a fluid. Such conventional extrusion of plastics is cumbersome and difficult to carry out free of defects whenever the cross-section of the extruded product is large. This is due to the differences in cooling rates between cross-sectional regions of the product. For example, the plastic is slowest to solidify at the centerline of the extrudate and as a result voids tend to appear due to shrinkage during the cooling process causing defects in the resulting product. In the production of polyethylene bars with a diameter of 50mm or more, for example, due to the poor heat conductivity of the plastic, the center region of the extrudate cools very slowly even if the outside surface is vigorously cooled. It is thus near impossible to avoid the formation of shrinkage defects near the centerline.